1. Field of the Invention
The present invention relates to a process for reducing the molecular weight polydispersity of polyether glycols, particularly poly(tetramethylene ether) glycols, by membrane fractionation. More specifically, the present invention involves an improved process for recovering PTMEG characterized as having a narrow molecular distribution and an unusually low polydispersity achieved by batchwise or continuous removal of low molecular weight species using a cross-flow ultrafiltration or membrane separation technique.
2. Description of the Related Art
It has long been recognized that a narrow molecular weight distribution and/or a numerically low polydispersity (i.e., Mw/Mn where Mw is weight average molecular weight and Mn is the number average molecular weight) for poly(tetramethylene ether) glycols, PTMEG, is desirable for a number of end use applications in specialty polyurethanes and polyether esters based on the PTMEG building blocks; --O--CH.sub.2 CH.sub.2 CH.sub.2 CH.sub.2 O).sub.n. Consequently, various methods of altering or controlling the molecular weight distribution of PTMEG have previously been suggested with varying degrees of commercial success.
For example, Japanese Kokai No. 85/42421 discloses treating broad molecular weight distribution PTMEG with a mixture of water and a solvent having poor dissolving power for PTMEG while U.S. Pat. No. 3,358,042 employs water, steam distilling and a water-immiscible polymer solvent and U.S. Pat. No. 3,478,109 suggests methanol extraction after dissolving the polymer in an organic solvent. U.S. Pat. Nos. 4,762,951 and 4,933,503 disclose a three phase separation technique preceded in the later patent by a low pressure distillation step to achieve a narrower molecular weight distribution product. U.S. Pat. No. 3,925,484 teaches a method of narrowing the molecular weight distribution by partial depolymerization in the presence of an acidic ion exchange resin and U.S. Pat. No. 4,585,592 suggests treating with an oxygen-containing gas with each being followed by low pressure distillation. In U.S. Pat. Nos. 4,163,115 and 4,510,333 a method of using an acylium ion precursor to polymerize THF is disclosed with the latter maximizing the tertiary oxonium ions prior to the polymer propagation thus leading to a narrow molecular weight distribution.